In the past, coating compositions were commonly prepared by dissolving or dispersing film-forming organic polymers in volatile organic compounds (VOC). Environmental and health concerns associated with applications involving large-scale vapor emissions have led to research to provide coating compositions wherein the emission of volatile organic compounds is minimized.
Among the various methods of reducing the vapor emissions of coating compositions, the use of reactive diluents to replace all or part of the volatile organic solvent component of a coating composition is of particular interest. As used herein, the term "reactive diluent" refers to organic solvents or dispersants having as an integral part of their molecular structure, functional groups which are reactive with a film-forming polymer and/or cross-linking agent. The reactive diluent can thus be reacted to sufficiently high molecular weight to reduce its volatility and thus reduce volatile organic emissions from the coating composition.
In addition to providing a low level of volatile emissions, a coating composition should have a sufficiently low viscosity to permit easy handling and application. A reactive diluent should not, however, unduly adversely affect the sought performance properties of the coating composition or ultimate coating. These properties include sufficient stability to ensure a commercially acceptable shelf life and the ability to provide a cured coating having suitable properties such as toughness, adhesion, scratch resistance, weatherability, and resistance to attack by solvents, acids, bases and other chemicals.
Coating compositions wherein all or a portion of the volatile organic solvent component thereof is replaced by a reactive diluent are illustrated by the following patents:
U.S. Pat. No. 4,417,022, to Chang, et al., discloses coating compositions, the vehicle portion of which consists essentially of from about 60 percent to about 97 percent of a curable film-forming component and from about 3 percent to about 40 percent of an organic reactive diluent capable of chemically combining with the curable film-forming component. Disclosed as reactive diluents are ethers having less than five ether groups, amides, esters, urethanes, ureas, sulfur-containing compounds, and mixtures thereof which have one primary or secondary hydroxyl group. The reactive diluents disclosed by Chang, et al., are further characterized as having a retained solids value of greater than about 80 percent, a hydroxyl equivalent of from about 180 to about 800, and a liquid viscosity of less than about 10 poise at 60.degree. C. Preferred reactive diluents disclosed by Chang, et al., are ester-containing reactive diluents, with ester-containing reactive diluents having allyl side chains being most preferred. Coating compositions having ester-containing reactive diluents, however, tend to produce cured coatings which lack desirable adhesion, hardness and/or weatherability.
As a further example, U.S. Pat. No. 4,520,167 to Blank, et al., discloses a coating composition comprising (a) a hydroxyalkyl carbamate of the formula: ##STR1## wherein n is 0 or 1, R is a C.sub.1 to C.sub.20 organic moiety which may contain one or more constituents selected from the class consisting of hetero-atoms and hydroxyl groups, and each of R.sub.1, R.sub.2 and R.sub.3 is independently H or CH.sub.3 ; (b) an aminoplast (amide-aldehyde) cross-linker; and (c) a polymer containing active sites which at elevated temperatures are reactive with the amide-aldehyde cross-linker (b).
Among the compounds which have been offered for use as reactive diluents in coating compositions are dicyclopentenyl-oxyethyl ethyl methacrylates, modified caprolactones, and unsaturated melamines. In general, these compounds are poor solvents, have relatively high viscosities and/or produce cured coatings having undesirable chemical and/or physical properties.
U.S. Pat. No. 4,814,382 to Hoy, et al., discloses high solids coating compositions containing reactive diluent comprising a surfactant-like carbamate, urea and/or amide derivative. The compositions are said to provide desirable toughness and adhesion and generally exhibit uniform surface appearances. The patentees postulate that the improvement in surface appearance is due to enhanced wetting ability and/or flowability.
A cured coating's toughness, adhesion, impact resistance, abrasion resistance, scratch resistance, weatherability and resistance to chemical attack depend to a large extent upon the film-forming polymer and reactive diluent and cross-linker components of the composition used to produce same. Coating compositions containing carbamate and urea derivatives as reactive diluents, such as disclosed in U.S. Pat. No. 4,814,382, often provide cured coatings having desirable toughness and adhesion.
As indicated above, industrial use of coatings is being increasingly limited in the amount of volatile organic compounds it can release into the atmosphere by government regulations intended to protect the environment and those who apply the coatings. This increasing pressure for change has created an opportunity for new technologies with potential for lower VOC. As mentioned above, one of these is the use of reactive diluents or cosolvents in place of all or part of the traditionally used solvents. Many early attempts to provide reactive cosolvents or diluents have failed because the resulting coating compositions suffered from degraded application or end-use properties.